Variable frequency inverter power supplies have been improved by the use of current source power supplies. For example, see U.S. Letters Pat. Nos. 3,863,121 granted Jan. 28, 1975 and assigned to the same assignee as the present invention. Inverters have been built using a phase-controlled rectifier for control purposes. This is a simple, rugged control system that can tolerate abuse and yet continue operation when supplied from alternating current (AC) power mains. This is because the phase-control rectifier is commutated by the reversal of the applied AC voltage. If a misfire were to occur, the AC voltage continues reversing in polarity giving the control circuit another opportunity to fire on the next cycle. This can be readily accomplished with the proper sizing of power circuit elements.
Inverters can also be operated from direct current (DC) power mains by use of a chopper. As its name implies, a chopper provides an adjustable magnitude DC voltage by repetitively applying the constant voltage from the DC power main for adjustable time intervals so that over a long period the average value of the direct current is at a desired level. However, with a chopper as the controlling element, there is no source of energy for commutation like there is with the phase-controlled rectifier. The chopper depends upon stored energy to turn itself off. If for some reason a misfire occurs, or the stored energy is not enough to turn the chopper off, there is no second chance to commutate as there is with the phase-controlled rectifier. Yet it is desirable to use a DC-to-DC chopper to regulate the current in a current-source inverter. This gives the capability of producing output frequencies much higher than the 120 to 150 Hz limit when a phase-controlled rectifier is the current regulator.